Conventional train rail track structures are typically comprised of approximately 6 or more major components: (1) the steel rail, (2) the tie plate or “chair” that the rail sits on, (3) the railroad cross tie or “sleeper” to which two tie plates are affixed, (4) the fasteners that secure the rail to the tie plate and tie plate to the cross tie, (5) a means for adjoining consecutive lengths of rail sections and (6) the foundation or bed of ballast rock in which the cross ties are located and the track system is held in place.
In North America wood is the predominant cross tie material used in track structures. It is first treated with a preservative such as creosote, then field assembled with the rail via a metal tie plate and screw or spike fasteners through the tie plate into the wood. The list of problems with wood cross ties include splitting or cracking along the grain lines, spikes coming loose or working out from the tie plates, insect degradation, weather degradation, leaching of toxic chemicals into the ground water, air pollution when incinerated, loose tie plates that can result in rail gauge changes, shifting in the ballast bed under side loads, not being strong or stiff enough for high speed use and floating or being washed away when submerged by flooding waters.
Even with these operational problems, the wooden railroad cross tie still holds a dominant market share of about 90 percent in America. The remaining market share is comprised of precast concrete ties with a steel tie plate or chair cast into the upper surface of the cross tie, extruded or molded plastic ties with the metal tie plates being spiked or screwed into the plastic during field installation and the least common being stamped or forged steel ties with means for attaching the rail directly to the metal cross tie without the use of additional tie plates being screwed onto the steel cross tie.
Concrete is the second most common cross tie material in North America. Concrete cross ties are more prevalent in Europe and in high-speed passenger applications throughout the world. Concrete tie systems typically use over-center tension clips, such as made by the firm Pandrol, for affixing the rail to the molded-in metal tie plate on the upper surface of the concrete tie. Also, there are limited examples of the rail track structure being a monolithic concrete bed that uses no sleepers or ties whatsoever. These sleeperless or slab style installations are used for very high speed and tunnel applications where maintenance is difficult to perform or where thermal stresses from significant temperature variations are less prevalent. Concrete sleeper and slab style track systems are significantly more expensive to install than wooden tie systems but carry the expectations of increased safety and lower lifecycle costs relative to wooden tie systems. Concrete track structure and concrete cross ties have demonstrated less than expected life cycle duration with problems such as rail seat deterioration, cracking from cyclical or impact loads, surface water retention and freeze-thaw spalling.
Plastic or composite is the third most common cross tie material in North America. They are typically more expensive than wood and about the same or slightly higher in price than concrete. Plastic cross ties are difficult to manufacture in high volumes, have problems with surface cracking, have less resistance to side shifting than wood or concrete, are more flexible than wood, not as strong as wood or concrete and not desirable for high speed passenger service.
The predominant means for affixing consecutive sections of steel rail is butt-welding them together for a connected length being as much as a kilometer or longer. The single piece of welded rail is then installed onto the tie plates and independent cross ties. Continuous Welded Rail (aka CWR) or ribbon rail can be stronger than sectioned rail and can be less maintenance intensive. CWR is different than the historic method, which had a mechanical joint at every rail section. It is the steel wheels of the train cars rolling over the gaps in the section joints that result in the signature “clickity-clack” sound familiar to train transportation in the past.
There is an intrinsic and serious problem with CWR that does not occur with sectioned rail that uses conventional expansion joints. Steel rail expands in length when it is heated and contracts in length when cooled. This thermal expansion or contraction can be significant enough to cause rail track failure in hot conditions by twisting out of shape (aka sun kinks) and even snap in cold conditions (aka pull-a-parts). The thermal coefficient of expansion for steel is 0.00000645 inches of expansion/contraction per inch of length per degree of temperature change in Fahrenheit. For a one mile section of track and a 100-degree temperature variation the change in length for an unrestrained section of track is approximately 40.6 inches. It is possible for many rail track sections in the American Midwest to experience a 200-degree temperature variation over a 12-month period, due to the deep sub-zero F ambient winter temperatures and the elevated ambient summer temperatures plus radiant heat absorption from direct sunlight.
To avoid temperature related shifting of the track bed, physical distortion of the rail and interrupted service or derailment, it is preferred for CWR to be laid on concrete sleepers. The extraordinary weight of the concrete sleeper can be useful in holding the rail in place against the thermally induced stresses. CWR is frequently installed on wooden tie track systems, which creates persistent maintenance activity in anticipation of rail movement. During the hot summer months, train rail is routinely cut to relieve the built up stress and then re-welded. During the winter period the track has to be heated in order for pull-a-parts to be mended by re-welding. When new track is laid or whole sections replaced, it is usually heated to an estimated neutral temperature for that locale. This pre-heating technique attempts to minimize the expected repair work. The unsolved problem of thermal stresses in CWR systems requires persistent, ongoing repair. It is common practice for inspectors to walk or ride the track systems and physically examine the rail when the weather conditions warrant.
The rock ballast in the track bed has a tendency to settle and subside due to use over time, weathering effects, thermally induced loads and lateral forces on the as trains go through curve sections. All three primary railroad ties (concrete, wood and plastic) typically have a nominally rectangular, solid cross section. Maintenance of the ballast rock bed and keeping up the edges or shoulders of the ballast bed on the outside of the ends of the ties is vital. When the forces horizontal and perpendicular to the steel rail occur, it is the friction force due to the weight of tie and track plus any resistance by ballast rocks outside the tie ends that resists shifting. Since current sleeper designs are nominally rectangular and solid in cross-section, correcting subsidence of the ballast rock bed at the rail bed shoulder and keeping the ballast rock bed intact is an important maintenance function.
Concrete ties are considerably more expensive than wood, do not co-mingled with other tie types and more commonly found in new construction. Concrete ties, due to their weight, require different equipment for handling and installation than regular wood or plastic molded ties. Concrete ties are susceptible to stress cracking from the wheel loads moving across the tie and often have cushioning pads between the metal rail seat and the bottom of the steel rail. Concrete ties are steel reinforced to absorb the tension or bending loads that can occur on the ties. Concrete ties do not absorb vibrations as well as other ties. Concrete ties can have accelerated failure due to incorrect cement recipes, insufficient curing time or environmental degradation. Concrete ties do not attenuate the wheel to rail noise as well as wood or plastic.
Plastic cross ties are more expensive than wood and not readily available in large quantities. Plastic ties are more likely than concrete or wood to shift from side loads due to a lower weight and low relative coefficient of friction with the rock ballast.
Wood cross ties are the least expensive but have the shortest expected life cycle before needing replacement. Wood ties are more subject to weather related degradation. In certain locations like Africa wood ties cannot be used due to rapid destruction from insects like termites. Wood ties are more likely to release the spike or screws that hold the rail to the tie plate and thus subject to vandalism. The toxic preservatives used to extend the life of wood ties leach out over time and contaminate the environment.
The thermally induced stress in steel rail is a universal problem and well understood. Expensive and elaborate expansion joints with special clips like the Pandrol Zero Longitudinal Restraint can be currently found in the more vulnerable and valuable track sections of high-speed passenger lines, such as bridges, tunnels and curves. Full resolution of the thermal stress problem can be accomplished by the frequent use of expansion joints along the full track length. Utilizing current design expansion joints would greatly increase the installed costs of the already expensive concrete tie track systems. Increased cost is the primary barrier to solving this thermal expansion problem.